1. Field of the Invention
The present invention relates generally to the construction of electrodes, and more particularly to electrodes used to deliver electrical energy to the skin having a design and construction that is particularly suited to resist corrosion.
2. Description of Related Art
For many years electrodes have been used to deliver electrical energy to the skin for various purposes such as for pain management and muscle stimulation. When used for transdermal applications, stimulation electrodes usually require the use of a conductive liquid or gel, often called a “hydrogel”, to provide a continuous and efficient conductive path between the current source and the skin. Conductive hydrogels typically contain salt, and as such can be corrosive to common electrode trace materials, which adversely affects the performance of the electrode. Thus, when designing integrated electrodes for transdermal stimulation, in order for such devices to be commercially viable they must have a sufficiently long shelf life, which requires a design that minimizes or eliminates the ability of the hydrogel to migrate and reach the trace elements over time.
One known method for “isolating” a hydrogel from copper traces in an electrode is to cover the copper with an electrically conductive, but more corrosion resistant or inert substance, such as gold. An example of this is illustrated in FIG. 1, where electrode 100 includes a non-conductive substrate 101 (i.e., fiberglass) having an aperture 102 through which a conductive trace material or conductive pad 103 passes and is subsequently electrically coupled to an integrated circuit 120 or the like that provides power to the electrode. The conductive trace material is also applied across a top surface of the non-conductive substrate. A gold or nickel/gold layer 104 is then applied via the well established ENIG (electroless nickel immersion gold) process over the conductive trace element so as to isolate the trace material from the hydrogel 105 as described above. Gold is well known to be a conductive, but inert material, but is also well known to be expensive. Further, although a gold layer theoretically prevents corrosion, in reality variations and/or imperfection in manufacturing processes, particularly in thin film techniques, result in varying degrees of corrosion over time, which presents challenges when designing products for long-term use and/or when long term shelf-life is needed.
Others have been known to incorporate further additional conductive layers between the copper trace material and the hydrogel in an effort to prevent or minimize corrosion. This solution has been used by Alza Corporation of Mountain View, Calif., and an example of such solution is illustrated in FIG. 2. The non-conductive substrate 201 of the electrode 200 includes a conductive copper trace material 203 on its top surface. Deposited on top of the trace material 203 is one or more additional conductive, but inert or more corrosion resistant layers such as an electrically conductive tape 206 and a silver foil 207. When the electrode is placed on the skin of a patient, the hydrogel 105 is placed between the skin and the additional conductive players. Although incorporation of additional layers between the copper and hydrogel does provide added corrosion protection, it also increases the material and assembly costs for the electrode.
Accordingly, what is needed is an improved electrode design that has reduced material and assembly costs, yet provides sufficient corrosion resistance for use as a commercial, transdermal electrode assembly.